1. Field of the Invention
The present invention relates to an ink jet recording method and apparatus, which record an image on a recording medium by ejecting ink droplets according to image data.
2. Related Background Art
With spread of copying machines, information equipment such as wordprocessors, computers, and the like, and communication equipment, an apparatus for performing digital image recording using an ink jet recording head has become increasingly popular as one of recording apparatuses for such equipment. A recording apparatus of this type uses a head prepared by integrating a plurality of ink ejection orifices and ink channels as a recording head (to be referred to as a multi-head hereinafter) in which a plurality of recording elements are integrated and aligned for the purpose of an increase in recording speed. Furthermore, a color recording apparatus normally comprises a plurality of multi-heads.
Unlike in a monochrome printer for printing characters alone, when a color image is to be printed, various characteristics such as color development characteristics, gradation characteristics, uniformity, and the like are required. In particular, as for the uniformity, a small variation in units of nozzles, which is generated during a multi-head manufacturing process, influences the ejection amount or ejection direction of an ink of each nozzle, and consequently deteriorates image quality as a density nonuniformity of a printed image.
An example of the density nonuniformity will be explained below with reference to FIGS. 71A to 72C. In FIG. 71A, a multi-head 91 is constituted by eight multi-nozzles 92 for ejecting ink droplets 93. Normally, the multi-nozzles 92 ideally eject the ink droplets in a uniform amount and in a uniform direction, as shown in FIG. 71A. If such ejection is performed, dots having a uniform size land on a sheet surface, as shown in FIG. 71B, and a uniform image free from the density nonuniformity can be obtained as a whole (FIG. 71C).
However, in practice, each nozzle suffers from a variation, as described above. If a print operation is performed in the same manner as described above, ink droplets having various sizes are ejected from the nozzles in various directions, as shown in FIG. 72A, and land on a sheet surface, as shown in FIG. 72B. As shown in FIG. 72B, a blank portion, which does not satisfy an area factor of 100%, conversely, a portion where dots unnecessarily overlap each other, and a white line (at the center of FIG. 72B) periodically appear in the head main scan direction. The group of dots which landed in the state shown in FIG. 72B has a density distribution shown in FIG. 72C in the nozzle alignment direction, and consequently, such a phenomenon is normally observed as a density nonuniformity by human eye.
As a countermeasure against such density nonuniformity, the following method has been proposed. The method will be described below with reference to FIGS. 73A to 74C. According to this method, in order to complete a print area shown in FIGS. 71A to 72C, the multi-head 91 is scanned (main scan) three times, and a half area in units of four pixels is completed by two passes. In this case, the eight nozzles of the multi-head are divided into two groups respectively including upper four nozzles and lower four nozzles. Dots to be printed by one nozzle in a single scan are obtained by thinning out given image data to about a half according to a predetermined image data arrangement. In the second scan, dots corresponding to the remaining half image data are printed, thus completing the area in units of four pixels. The above-mentioned recording method will be referred to as a divisional recording method hereinafter.
When such a recording method is used, even if a head equivalent to the multi-head shown in FIG. 72A is used, since the influence of the nozzles to a printed image is reduced to half, an image shown in FIG. 73B is printed, and black and white lines observed in FIG. 72B do not become conspicuous. Therefore, the density nonuniformity is remarkably eliminated as compared to FIG. 72C, as shown in FIG. 73C.
Upon execution of such recording, image data is divisionally thinned out to predetermined complementary arrangements in the first and second scans. As the image data arrangement (thinning pattern), a checker pattern in which dots are printed on every other pixels in the vertical and horizontal directions is normally used, as shown in FIG. 74A. Therefore, a unit print area (in units of four pixels) is completed by the first scan for printing dots in a checker pattern and the second scan for printing dots in a reverse checker pattern.
FIGS. 74A, 74B, and 74C explain how to complete a predetermined record area using the checker and reverse checker patterns by the multi-head having eight nozzles like in FIGS. 71A to 73C. In the first scan, dots are recorded in a checker pattern using lower four nozzles (FIG. 74A). In the second scan, a sheet is fed by four pixels (1/2 the head length), and dots are recorded in a reverse checker pattern .smallcircle. (FIG. 74B). Furthermore, in the third scan, the sheet is fed by four pixels (1/2 the head length), and dots are recorded in the checker pattern again (FIG. 74C). In this manner, when the sheet feed operation in units of four pixels, and recording operations of the checker and reverse checker patterns are alternately performed, a record area in units of four pixels is completed for each scan.
As described above, since the print area is completed by two different groups of nozzles, a high-quality image free from the density nonuniformity can be obtained.
Such a recording method has already been disclosed in Japanese Laid-Open Patent Application No. 60-107975 and U.S. Pat. No. 4,967,203, and these references describe that this method is effective to remove the density nonuniformity and connection lines. The former reference discloses that "the invention is characterized by comprising means for forming an overlapping portion by overlapping two adjacent main scans by setting a sheet feed width of each main scan to be smaller than the width of the main scan, and means for printing dots of the overlapping portion so as not to overlap each other in the two main scans". According to this reference, as described above, a thinning mask is defined as one for "alternately printing odd and even rows in every other columns" in one case. However, in another case, odd rows are printed in the first scan, and even rows are printed in the second scan. In still another case, odd and even rows are randomly printed in each scan. Thus, the thinning mask and the sheet feed width are not completely limited.
In contrast to this, the latter U.S. Pat. No. 4,967,203 discloses that
"a) in the first pass, dots are printed at alternate pixel positions, which are not two-dimensionally adjacent to each other, of only the upper half of a first band, PA1 b) in the second pass, dots are printed on pixel positions, which are not printed in the first pass, in the first band, and at alternate pixel positions, which are not two-dimensionally adjacent to each other, in the lower half of the first band, and PA1 c) in the third pass, dots are printed at pixel positions, which are not printed in the first and second passes, in the first band, and at the same time, the first pass print operation in the next band." In this manner, in this reference, a thinning mask used in divisional recording is limited to an alternate pixel arrangement in which pixels are not two-dimensionally adjacent to each other.
As an arrangement to be additionally described in this reference, a recording method wherein a pseudo pixel (super pixel) as a group of several pixels is formed for the purpose of gradation expression and multi-color expression, and an alternate thinning print operation at two-dimensionally non-adjacent pixel positions in units of pseudo pixels (super pixels) is disclosed. It is described that according to this method, "once a system for realizing the method is installed in a software program or printer firmware, since the program can be called by the numbers of color combinations designated in association with super pixels, high print quality can be achieved without unnecessarily complicating an operation for creating a computer program for generating a large number of colors", and simplified programming for achieving multi-color expression is listed as one effect. It is also described that since each super pixel is intended to be perceived as a single uniform color, color blurring within each super pixel is harmless.
The above-mentioned divisional recording requires considerable time cost per page, and the throughput is inevitably lowered. In this case, in order to shorten the print time, a method of reciprocally print-scanning a carriage is proposed. According to this method, since all carriage scans each for returning the carriage to the home position without performing any record operation after one record scan can be omitted, the record time per page can be reduced to almost half. In practice, the reciprocal print operation is popularly adopted as a monochrome print method. However, in a color ink jet apparatus having the arrangement of the present invention, the reciprocal print method is not put into practical applications yet for the following reasons.
FIG. 75 is a sectional view of a normally used recording ink and a landing state of the ink printed on a medium (paper sheet). FIG. 75 illustrates a state wherein two different color inks (dots) are absorbed (recorded) at almost neighboring positions to have a time distance therebetween. It is to be noted that, in an overlapping portion of two dots, the subsequently recorded dot tends to extend under the previously recorded dot in the sheet depth direction. Such a phenomenon is caused for the following reason. That is, in a process wherein a dyestuff such as a dye in the ejected ink is physically and chemically coupled to a recording medium, since the coupling capacity between the recording medium and the dyestuff is finite, the previously ejected ink dyestuff is preferentially coupled to the recording medium as along as there is no large coupling force difference depending on the types of dyestuffs, and remains in a large amount near the surface portion of the recording medium. Conversely, the subsequently ejected ink dyestuff is not easily coupled to the surface portion of the recording medium, and is fixed after it sinks deep in the sheet depth direction.
In this case, even when two different inks are printed at a single landing point, a priority color varies depending on the print order of the two different inks, and consequently, two different colors are expressed for visual characteristics of man. For example, assume that four color heads are arranged in the order of black, cyan, magenta, and yellow from the right, and main scans are performed by reciprocally moving the heads in the head alignment direction (right-and-left direction). In a forward scan, the heads are moved rightward, and simultaneously perform recording. At this time, since the recording order on a sheet surface follows the alignment order of the heads, for example, when a green (cyan+yellow) signal is input to a given area, inks are absorbed by each pixel in the order of cyan and yellow. Therefore, as described above, in this scan, the previously absorbed cyan serves as the priority color, and a cyanish green dot is formed. Conversely, in a backward scan after a sheet feed operation is performed in the sub-scan direction, the heads perform recording while being moved in a direction opposite to the forward scan. Therefore, the print order is reversed, and in this scan, a yellowish green dot is formed. When such scans are repeated, cyanish green dots and yellowish green dots are recorded according to the forward and backward movements of the recording heads. If each scan does not use the divisional print method and the sheet feed operation is performed by the head width after each of the forward and backward scans, a cyanish green area and a yellowish green area alternately appear by the head width, and a green image which should be a uniform image, is considerably deteriorated.
However, this defect can be slightly conquered using the conventional divisional recording method. More specifically, in the divisional recording method, as has been described above with reference to FIGS. 74A to 74C, cyanish green dots are recorded in the forward scans (FIGS. 74A and 74C), and yellowish green dots are recorded in the backward scan (FIG. 74B). Therefore, the color tone of a given area is relaxed by the dots having the two different color tones.
The arrangement and effect for eliminating color nonuniformity in units of bands by mixing dots having two different color tones in a given area, as described above, have already been disclosed in U.S. Pat. No. 4,748,453. In this reference, although the sheet feed amount is not limited, dots are complementarily recorded at two-dimensionally alternate pixel positions in two (first and second) or more record scans, thereby preventing beading of inks on a medium such as an OHP sheet. In addition, when a color image is recorded, the ink landing order for color-mixed pixels is reversed between the first and second scans (reciprocal recording), thereby preventing color banding (color nonuniformity). Since this reference has as its principal object to prevent beading between neighboring pixels, it is characterized in that dots are recorded at two-dimensionally alternate (non-adjacent) pixel positions in a single scan.
Japanese Laid-Open Patent Application No. 58-194541 by the same applicant (Canon K. K.) as the present invention discloses a technique wherein a plurality of recording element arrays are arranged parallel to each other, upon execution of a main scan for recording a dot matrix by reciprocally moving the recording element arrays in a direction perpendicular to the recording element arrays, dots fewer than all dots in at least one of rows and columns of the recording dot matrix are intermittently recorded in a forward main scan, and remaining dots in at least one of rows and columns of the recording dot matrix are intermittently recorded in a backward main scan, so that the forward and backward main scans have different overlay recording orders of overlay recording dots using the plurality of recording element arrays. In this reference as well, there is no limitation such that the sheet feed width is set to be smaller than a normal width unlike in the previously described divisional recording, and the effect of this reference is to prevent deterioration of image quality caused by color mis-registration (color nonuniformity) of a recorded image caused by overlay recording of color inks.
Since this reference has as its principal object to prevent color mis-registration, dot positions to be recorded in each scan are not particularly limited. In the embodiments of this reference, a horizontal thinning pattern used for alternately recording dots in only the vertical direction, and a vertical thinning pattern alternately repeated in only the horizontal direction are described in addition to checker patterns (checker and reverse checker patterns).
Also, Japanese Laid-Open Patent Application No. 55-113573 discloses an arrangement for performing reciprocal recording using checker patterns (checker and reverse checker patterns) although it is not limited to a color printer. This reference inhibits continuous print operations of neighboring dots, thereby preventing a dot distortion caused by printing a neighboring dot before a previously printed dot is dried. Therefore, in this reference, a thinning mask is limited to a checker pattern like in U.S. Pat. No. 4,748,453.
The three references presented above have as their objects to prevent color nonuniformity and beading in reciprocal recording. Therefore, these references do not employ an arrangement in which "the sheet feed amount between adjacent scans is set to be equal to or smaller than a normal head width", which arrangement is employed for the purpose of preventing the density nonuniformity caused by variations of nozzles, unlike in the divisional recording method described in this specification.
As described above, when the divisional recording method is adopted in reciprocal recording, since two different groups of recording pixels formed in the opposite ejection orders of color inks can be uniformly arranged in a record area, it is expected that multi-color bi-directional recording, which easily causes color nonuniformity normally, can be realized.
However, even when the above-mentioned divisional recording using the checker and reverse checker patterns is performed, the defect of color nonuniformity is not perfectly removed yet. The reason for this will be described below. In general, the amount of an ink droplet is designed so that ink spreads wider than an area for each pixel on a sheet surface. This is to eliminate any blank portion in an area corresponding to a print duty of 100%. Therefore, even when the divisional recording method is executed, although recording pixels themselves are printed at only 50%, an almost 100% area of a recording medium (recording sheet) is covered by dots, as shown in FIG. 76. FIG. 77 is a sectional view of the sheet surface in this case. In FIG. 77, a checker print operation is performed on a blank sheet in the first pass (forward scan), and a reverse checker print operation is performed in the second pass (backward scan). Reference numeral 2001 indicates a state of inks immediately after the print operation in the first pass (forward scan). In this state, a solid black portion represents a cyan ink, and a hatched portion represents a yellow ink. Since the yellow and cyan inks are printed at an identical position to have a very small time distance therebetween, when they are absorbed by the sheet, the cyan ink is less blurred in a high-density state, and the yellow ink is largely blurred to extend to portions under and around the cyan ink in a low-density state. Also, at this time, the absorbing range of these inks extends over neighboring pixel positions, and as shown in FIG. 76, almost the entire sheet surface is filled with the ink dots.
In the second pass (backward scan) performed under this condition, dots land on the sheet surface on which neighboring ink dots are absorbed, as indicated by reference numeral 2003. Since the second pass is a backward scan, the yellow ink is printed first, and the cyan ink is printed second (2002). When the inks are absorbed in this state, an absorbing state in which both the colors do not clearly appear on the surface is finally formed, as indicated by reference numeral 2003. In a completed image, the density of the cyan ink, which was printed first, is emphasized most strongly, and a green image having cyan as a priority color tone is formed on this print area. Conversely, in a print area which has a backward scan as the first pass, and is adjacent to the above-mentioned print area, the situations of the cyan and yellow inks are reversed, and a green image having yellow as a priority color tone is formed.
FIG. 78 illustrates a state wherein the above-mentioned two print areas appear. As can be seen from FIG. 78, the lower half nozzles of the heads determine the priority color of each area, and the priority color is reversed between the forward and backward scans. Since two areas having the different priority colors are alternately formed, color nonuniformity still appears in the divisional print method, and deteriorates an image, thus preventing practical applications of the reciprocal print operations.
Most of image data are sent as actual signals after multi-value data representing certain gradation levels are binarized by a predetermined binarization method such as a dither method to have a predetermined pattern. Therefore, the number of pixels recorded in the first pass may be considerably different from the number of pixels recorded in the second pass depending on a thinning mask.
Such a phenomenon will be described below with reference to FIGS. 81(A) to 84. In this case, four multi-heads each having eight nozzles are used. The four colors are cyan (c), magenta (m), yellow (y), and black (k). As recording image data, an intermediate color (yellowish green) image obtained by overlaying the c and y inks respectively at print duties of 62.5% and 100% as shown in FIG. 81(A) is printed. A pixel indicated by a pin stripe pattern is a pixel on which c and y dots are printed, and a pixel indicated by a hatching pattern is a pixel on which only a y dot is printed. When the intermediate color shown in FIG. 81(A) is to be printed using a checker mask, c and y dots are printed at a duty of 50% on all possible pixel positions allowed by the checker mask (FIG. 81(B)) in the first pass. In the second pass, c and y dots are printed at the remaining duties, i.e., respectively at duties of 12.5% and 50%. When these passes shown in FIGS. 81(B) and 81(C) are observed in units of recording heads (colors), the c and y heads respectively eject the inks, as shown in FIGS. 81(D) and 81(E), and FIGS. 81(F) and 81(G).
FIG. 82 illustrates the ejection positions of the c and y recording heads in the first scan of the divisional recording method, and a dot formation state on a recording medium as a result of recording. In FIG. 82, a pin stripe pattern represents that both the c and y heads eject the inks on an identical pixel, and a hatched pattern represents that only the y head ejects the ink. In the first scan, each recording head uses four nozzles in a record section (1), and records dots in a checker pattern. As a result, dots in each of which the c and y inks overlap each other are formed in the checker pattern on the recording medium. The sheet is fed by an L/2 width, and the dots recorded in the first scan are moved to a record section (2).
FIG. 83 illustrates the ejection positions in the second scan, and a dot formation state on the recording medium as a result of recording.
In this scan, each head prints dots in a reverse checker pattern using both the record sections (1) and (2). As a result, dots formed by the record section (2) overlap the dots recorded in the checker pattern in the first scan, thus completing recording. The sheet is fed by another L/2 width, so that the dots formed by the recording section (2) are moved outside the record section, and the dots formed by the record section (1) are moved to the record section (2).
As described above, when a dot is printed to overlap the previously recorded dot, the subsequently recorded dot tends to extend under the previously recorded dot in the sheet width direction in the overlapping portion (FIG. 75).
Therefore, FIG. 83 expresses the overlapping portions, so that the dots recorded in the second scan extend under the dots recorded in the first scan.
FIG. 84 illustrates the ejection positions in the third scan, and a dot formation state on the recording medium as a result of recording.
In this scan, each head prints dots in the checker pattern opposite to the second scan using both the record sections (1) and (2). As a result, dots formed by the record section (2) overlap the dots recorded in the reverse checker pattern in the second scan, thus completing recording.
However, at this time, the portion corresponding to the record section (2), and the portion outside the record section, on which recording has been completed in the second scan, have different color tones, and cause color nonuniformity although they have same ejected ink amount.
A cause for this phenomenon is considered as follows. That is, since the dots are formed in the checker pattern first on the portion outside the record section, many dots formed by ejecting the c and y inks on identical pixel positions are present on the surface portion of the recording medium. In contrast to this, on the portion corresponding to the record section (2), since many dots formed by only the y ink are present on the surface portion of the recording medium, this portion forms yellowish green in which the yellow color tone is relatively strong.
As a method of eliminating the above-mentioned defect, a method wherein the number of pixels per color landed in each scan is averaged by adopting the divisional thinning arrangement, which is asynchronous with the arrangement of image data for gradation expression in an area gradation method, so as to eliminate a color tone difference of the scans, is proposed. As shown in FIGS. 85 and 86, in, e.g., a so-called Bayer type area gradation method of the dither method, thinning patterns 601 and 602 shown in FIG. 86 are used in place of thinning patterns 501 and 502 shown in FIG. 85, so that the first and second passes have the same number of landed pixels, thereby obtaining a good image.
However, such a method cannot be applied to the bi-directional print method, which is expected to further improve the throughput. On the contrary, color nonuniformity appears even on a color-mixed image, which is obtained by mixing two colors at the same duty, and does not pose any problem in the one-directional print method. Such a phenomenon is caused since the alignment order of the heads with respect to the moving direction of the carriage is reversed between the forward and backward scans, and hence, the ink landing order is completely reversed between the forward and backward scans.
The fundamental factor depends on a blurring state occurring when two different color inks land on an identical pixel position, as has been described above. However, such a phenomenon appears not only when two different color inks are printed on an identical pixel position, but also when dots printed on neighboring pixel positions are blurred considerably. This phenomenon appears more conspicuously as the pass interval is increased. This is because such a blurring phenomenon depends on the ink absorbing state of the sheet surface when the second dot lands. That is, a landing state obtained when the second dot is printed on the sheet surface on which the ink is completely absorbed is different from a landing state obtained when the second dot lands on a position adjacent to the first dot before the first ink droplet is completely absorbed, i.e., before the absorbed ink is blurred. In the latter case, the landing states of the two dots are similar to each other.
An example of such phenomenon will be described below as a bi-directional 2-pass print method of an ink jet recording apparatus. For the sake of simplicity, a state of a green image obtained by printing cyan and yellow dots at duties of 100% within a 4.times.4 matrix will be exemplified. FIGS. 87 and 88 are views for explaining the conventional bi-directional print method, and illustrate the landing states of color dots in the first pass (forward scan) and the second pass (backward scan) when thinning masks 201 and 202 are used.
The mask pattern 201 is used in the first pass, and the mask pattern 202 is used in the second pass. In the same pass, cyan and yellow dots, and even other color dots are printed using the same mask. A print state to be obtained when green image data 203 is input to all pixels will be described below.
The heads are aligned in the order of black, cyan, magenta, and yellow with respect to the forward moving direction of the carriage. Therefore, in the first pass (forward scan), in order to form the green image 203, cyan dots are printed first, and yellow dots are printed after a short delay time (204). Conversely, in the second pass (backward scan), yellow dots are printed first, and thereafter, cyan dots are printed (205).
In FIG. 88, a state 401 corresponds to an ink state immediately after the first pass print when viewed in a sheet section. In the state 401, a solid black portion represents the cyan ink, and a hatched portion represents the yellow ink. Since the yellow ink is printed at the same position as the cyan ink to have a very small time distance, it lands to overlap the cyan ink. When these inks are absorbed by the sheet in this state, a state 402 is attained. Since the cyan ink lands before the yellow ink to contact the sheet surface, it is less blurred, and has a high density. However, the yellow ink to be absorbed immediately after the cyan ink is absorbed on the sheet surface on which the cyan ink has already permeated. For this reason, the yellow ink is largely blurred to extend under and around the cyan ink, and has a low density. The state 204 in FIG. 87 corresponds to this print state when viewed from above the sheet surface. In the state 204, a capital letter represents an ink color having a high density and serving as a priority color, and a small letter represents an ink color having a low density. In this case, the high-density cyan (C) dots and the low-density yellow (y) dots are printed at pixel positions of the mask 201 in the first pass. At this time, the absorbing range of these inks extends to the neighboring pixel position, as can be seen from the state 402, and in this state, the sheet surface is almost filled with the ink dots.
In the second pass performed under this condition, the ink dots are printed on the sheet surface on which neighboring ink dots have already been absorbed, as indicated by a state 403. In this case, since the reciprocal print operations are to be executed, the yellow ink is printed first, and the cyan ink is printed later in the second pass. When the inks are absorbed in this state, finally, they do not clearly appear on the surface portion, as indicated by a state 404. Therefore, as indicated by the state 205 as well, both the cyan and yellow inks are largely blurred and have a low density. Thus, in a complete printed image, the density of only the cyan ink printed in the first pass is emphasized, and a green image having cyan as a priority color tone is formed on this print area (206).
Conversely, in a print area adjacent to the above-mentioned print area, in which the mask for the backward scan (the second pass for the above-mentioned print area) is used in the first pass, the situations of the cyan and yellow inks are reversed, and a green image having yellow as a priority color tone is formed. Such two different print areas are alternately formed, color nonuniformity appears and deteriorates image quality, thus preventing practical applications of the bi-directional print method.
Therefore, in a printer designed for both a normal paper sheet and a coating paper sheet, only a one-directional print operation is performed in correspondence with the coating paper sheet although the printer can further improve the throughput for a normal paper sheet.
Furthermore, the defect caused by blurring of an ink to the neighboring pixel position is observed not only as the color nonuniformity but also in monochrome reciprocal print operations. Such a defect will be explained below. FIG. 79 shows the absorbing state of a monochrome ink in the first and second passes like in FIG. 77. In FIG. 79, a state 2101 represents a landing state immediately after the first pass print, and states 2102 and 2103 represent landing states after the second pass print when viewed in a sheet section. In the state 2102, the second pass print is performed immediately after the first pass print, and in the state 2103, the second pass print is performed after a certain delay time after the first pass print. These two states cause different absorbing states of the ink recorded in the second pass to the sheet surface. That is, in the state 2102, the ink is absorbed deep in the sheet depth direction, while in the state 2103, the ink printed in the second pass extends on the sheet surface. These states are also confirmed from the rear sheet surface side. That is, the ink in the state 2103 considerably penetrates the sheet to the rear surface side as compared to the state 2102. These states also appear as a density difference on the sheet surface (2104 and 2105).
The time distance generated by reciprocally scanning the carriage is sufficient with respect to the order of the time difference that causes the density difference between the above-mentioned states. This factor appears as a new defect upon execution of the reciprocal print operations. This defect will be described below with reference to FIG. 80.
In FIG. 80, the head performs a forward scan in the direction of an arrow from a position 2201 to perform recording corresponding to a first scan width. After the head performs recording for one line, a sheet is fed by a width 1/2 the scan width, and the head then performs a backward scan in the opposite direction in turn from a position 2202 shown in FIG. 80. Furthermore, after the sheet is fed by the same width as described above, the head performs the forward scan again from a position 2203 to perform recording in the direction of the arrow. Recording intervals of the second pass at positions 1 to 6 of the print area completed at this time are compared. More specifically, at positions 3 and 4, after the first pass print is completed, the second pass print is performed immediately after the sheet is fed by a 1/2 width. In contrast to this, at positions 1 and 6, after the first pass print, the second pass print is performed after an elapse of a time required for reciprocally scanning the carriage once. At positions 2 and 5, the two print operations are performed at just an intermediate time distance. Therefore, as has already been described above with reference to FIG. 79, the positions 1 and 6 have the highest density, the positions 2 and 5 have the next highest density, and the positions 3 and 4 have the lowest surface density since the ink is absorbed deepest. Therefore, the density nonuniformity appears on the left-hand side area where the positions 1 and 4 repetitively appear at an interval of the 1/2 width in the vertical direction, and on the right-hand side area where the positions 3 and 6 repetitively appear at the interval of the 1/2 width in the vertical direction, thus deteriorating image quality.
As described above, the blurring state to non-print pixel positions in the first pass causes dependency of the density on the recording interval between the first and second passes, and it can be understood from this respect as well that actual applications of the reciprocal print method have been impossible so far. In the above description, monochrome recording has been exemplified. This phenomenon also appears together with color nonuniformity in mixed-color recording, as has already been described above, and in this case, it is recognized as right and left different color nonuniformity portions or different color tones.
In one-directional recording as well, the following factor is known as a defect influencing the recording time distance. When the recording apparatus performs a head recovery scan to maintain its own driving scans during recording or waits for transfer of recording data, the carriage is temporarily set in a rest state. Such a rest state causes density nonuniformity which occurs irregularly on the order still larger than that of the time distance nonuniformity described above. More specifically, when the carriage is set in a rest state after the first pass print is completed, and the second pass print is performed after some time distance, a corresponding record area has a higher density than other areas. The density nonuniformity caused by such a factor will be referred to as rest nonuniformity to be distinguished from the time distance nonuniformity.
As described above, when the divisional recording or the bi-directional print method is realized to achieve high image quality and high-speed image formation in an ink jet recording apparatus for performing image formation by scanning recording heads in a direction different from the nozzle alignment direction of one head, image defects such as color nonuniformity, rest nonuniformity, and time distance nonuniformity remain unremoved.
As a recording medium for such an ink jet recording apparatus, a special-purpose paper sheet having a coating layer, which is manufactured in consideration of color development characteristics, fixing characteristics, and the like of an ink, is known. In recent years, however, demand for use of various media such as a normal paper sheet, TP paper for an OHP, and the like is increasing. With this demand, the recording apparatus itself is demanded to achieve adaptation (high image quality) to various media. However, since ink adaptability varies depending on media, when the above-mentioned print method is used for all media, some media may suffer from various defects. In particular, the TP paper has a particularly low ink absorption speed as compared to other media, and the affinity force between ink droplets, which land on the sheet surface but are not completely absorbed, overcomes the absorption force of the coating layer on the TP paper. Therefore, when two or more such ink droplets are present at neighboring positions, as shown in FIG. 89, the ink droplets draw each other, and are coupled to form a large ink droplet at a position displaced from their original landing positions. Such a phenomenon is unique to a recording medium such as the TP paper having a low ink absorption speed, and is called "beading".
Such a phenomenon does not always occur in units of dots. The recording state of an area recorded by simultaneous scans by a conventional serial type print method in which a recording head having a predetermined recording width is moved in the main scan direction, and a sheet is fed in the sub-scan direction after the main scan is completed will be examined below in respect to a wider area with reference to FIGS. 90A to 90C. If ink droplets are printed at a low density (low duty) so as not to contact each other, they do not influence each other, and can maintain independent states so that the center of each dot is located at the center of the pixel. However, when ink droplets are printed at a high density (high duty) so as to contact each other, each ink droplet is apt to join contacting neighboring ink droplets. Therefore, a series of ink droplets located at the end portion are drawn to the center of the scan width of the recording head by a very strong affinity force, and the density at their real landing positions becomes considerably lower than the density to be obtained (FIG. 90A).
After the above-mentioned first scan, the second scan is performed on an area contiguous with the first scan area. In this area as well, the affinity force of ink droplets acts, and ink droplets at the end portion are drawn to the center of the second scan width by the same phenomenon as that in the first scan described above. As a result, the density of the end portion is lowered (FIG. 90B).
When such scans are continuously performed, the end portions having a low density are formed adjacent to each other between two each adjacent recording areas of the first and second scans, and a plurality subsequent scans. These end portions form white lines between two each recording areas, and impair the uniformity of the entire image, thus considerably deteriorating image quality (FIG. 90C).
A recording medium such as TP paper having a low ink absorption speed has been described. Even in a recording medium such as coated paper having high ink absorbency, an image defect occurs on a connection area between two adjacent scans although a phenomenon in this case is different from the above-mentioned one. Even in general paper, since some sheets have poor ink absorbency like in the TP paper but some other sheets have absorbency equivalent to the coated paper, an image defect inevitably occurs on a connection area.
Since the coated paper is designed to quickly absorb an ink in the sheet surface, no ink drawing phenomenon (e.g., beading) occurs unlike in the TP paper. However, a density difference appears depending on the ink landing/absorption timing. FIG. 91A is a sectional view showing a state wherein an ink droplet of a predetermined amount lands on and is absorbed by the sheet surface, and FIG. 91B is a sectional view showing a state wherein two ink droplets obtained by equally dividing the above-mentioned amount land on and are absorbed by the sheet surface at a time interval. As can be seen from FIGS. 91A and 91B, if the ink amount remains the same, when the ink is divisionally recorded several times in a small amount each at a time interval, the ink tends to remain more on the surface portion of the sheet, and hence, the surface density is increased. Such a phenomenon also appears on a connection area between adjacent record scans.
FIG. 91B shows a state wherein images on two image areas are completed by two scans. Since an ink dot is designed to have a size larger than one pixel area, two or more dots overlap each other on all areas. At a boundary portion, since the extrusion portion is divisionally recorded by two scans at a time interval, a connection line having a higher density than that of other areas is undesirably formed. Also, the time interval between adjacent record scans is not always constant due to a different data transfer time, a recovery operation of the recording apparatus main body, and the like. When the recording head stands by at the home position after some record scans are completed, the density of the connection area is further increased, and image quality is further deteriorated. The density nonuniformity appearing in such a carriage rest state will be referred to as rest nonuniformity hereinafter.
When the above-mentioned divisional recording method is used, the defects described above on TP paper, coated paper, and the like can be avoided to some extent. More specifically, according to the divisional recording method, since an image in a single area is completed by two different groups of nozzles, the density nonuniformity in the single image area can be prevented, while a connection line in units of scans using neighboring nozzles at the end portions can be slightly moderated by the divisional recording method. On the TP paper, since a boundary portion of an image area on which record scans have been completed is contiguous with an area on which recording has been completed at least at a duty of 50%, an ink affinity force in a single image area is not so strong. On the coated paper, since two record scans are performed for each area at a time interval, the density is increased as a whole, and an increase in density of the connection area alone can be prevented.
With the above-mentioned divisional recording method, it has been attempted to form an image free from a sheet feed connection line, blurring at a boundary between different colors, nozzle nonuniformity, or beading on a recording medium such as TP paper having a low ink absorption speed or on a medium such as coated paper having a high ink absorption speed.
However, even in the divisional recording method for executing the two-divided record scans, especially, the sheet feed connection line cannot be satisfactorily eliminated. In particular, beading or a sheet feed connection line is far from being removed by the divisional recording method using as few as two passes. A state of a connection line appearing upon execution of the divisional recording method will be explained below.
FIGS. 92A to 92C are views for explaining a divisional print operations for completing an image by two scans. In FIGS. 92A to 92C, an image is recorded by a head consisting of eight nozzles, for the sake of simplicity. In a first record scan (FIG. 92A), dots are recorded on a first image area in a checker pattern at a duty of 50%, as shown in FIG. 92A. At this time, no dots are recorded on a second image area. If dots are recorded at neighboring positions, they draw each other, and cause, e.g., beading, as has been described above with reference to FIG. 89 or FIGS. 90A to 90C. However, in this case, since the dots are recorded at non-neighboring positions, they remain at real pixel positions. In FIGS. 92A to 92C, assume that these ink droplets are not completely absorbed by the medium yet due to the low absorption speed of the recording medium, and remain on the surface as liquid droplets.
In a second record scan (FIG. 92B), dots are recorded at pixel positions on the first image area, which positions are not subjected to recording in the first record scan. At the same time, dots are recorded on the second image area at non-neighboring positions, as shown in FIG. 92B. At this time, the dots recorded on the second image area maintain their pixel positions without drawing each other like in the first record scan. However, on the first image area, since the absorption speed of the medium is low, the ink droplets recorded in the first record scan still remain on the medium surface as liquid droplets. Therefore, in the current second record scan, since the ink droplets are recorded at positions adjacent to the above-mentioned remaining ink droplets, these ink droplets draw each other, and tend to move toward the center of the first image area. The affinity force of the first image area influences a pixel array, contiguous with the first image area, in the second image area. Dots on this pixel array are adjacent to the dots at the end portion of the first image area at an instance when the second record scan is performed and the first image area is completed, and are drawn toward the first image area by the strong affinity force of the completed first image area. Therefore, dots on the pixel array, closest to the first image area, of the second image area, are displaced from their real pixel positions, and this portion forms a white line having a low density.
A third record scan will be described below. In this scan, dots are recorded at non-neighboring pixel positions in a third image area while completing the second image area, like in the second record scan. The ink affinity force is generated in the second image area like in the second record scan, and ink droplets in the second image area and inks at the end portion of the third image area are drawn toward the center of the second image area (FIG. 92C). At this time, a pixel array at the end portion on the first image area side in the second image area is adjacent to not only those in the second image area but also to a non-absorbed pixel array in the first image area. Therefore, both a force toward the center of the second image area and a force toward the center of the first image area act on this pixel array at the end portion. However, the first image area includes the ink droplets recorded in the first record scan, and the second image area includes ink droplets currently recorded in the third record scan. Therefore, since the second image area has a larger absolute amount of non-absorbed ink droplets on the medium than the first image area, the force toward the second image area is stronger than the force toward the first image area, and the dots at the end portion are drawn toward the second image area. Therefore, a connection white line is formed between the completed first and second image areas.
Such a pixel drawing phenomenon at the end portion caused by an ink affinity force is gradually weakened as the number of divisions of the divisional recording method is increased. However, as long as the divisional recording is performed, adjacent image areas during printing inevitably have a difference in degree of print completion more or less. At an instance when an area having a high degree of print completion has reached a duty at which dots are recorded at neighboring positions, the area draws dots at the end portion of the neighboring area having a low degree of print completion, and forms a white line. Therefore, an increase in the number of divisions can never be a fundamental solution to the connection lines. On the contrary, if the number of divisions is increased, since the number of record scans per unit area is increased, another problem, i.e., an increase in recording time cost, is posed.
On coated paper, the divisional recording method worsens image quality in terms of rest nonuniformity. In the divisional recording method in which a single image area is completed by two scans, if a rest time is inserted between the two scans, the density of the corresponding image area is undesirably increased, and a high-image density band appears in a recorded image.
As described above, no image which can satisfactorily eliminate connection lines can be obtained on a medium such as TP paper having a low ink absorption speed and on a medium such as coated paper having a high ink absorption speed.